Stress Shadows: Insights into Physical Models of Aftershock Triggering

Why some aftershocks appear to occur in stress shadows, regions of Coulomb stress decrease due to a mainshock, is an open question with implications for physical and statistical aftershock models. New machine-learning focal mechanism catalogs make it possible to study the fault orientations of aftershocks occurring in the stress shadows, and test competing hypotheses about their origins. There are three main hypotheses: (1) Aftershocks appear in shadows because of inaccuracy in the computed stress change. (2) Aftershocks in the shadows occur on faults with different orientations than the model receiver faults, and these unexpected fault orientations experience increased Coulomb stress. (3) Aftershocks in the shadows are triggered by dynamic stress changes. We test these three hypotheses on the 2016 Kumamoto and 2019 Ridgecrest sequences. We test Hypothesis 1 through many realizations of the stress calculations with multiple mainshock models, multiple receiver fault orientations based on background events, and a range of coefficients of friction. We find that numerous aftershocks are consistently in the stress shadows. To test Hypothesis 2, we consider whether the individual event focal mechanisms receive an increase of Coulomb stress. Again, we perform many realizations of the stress calculation, this time with receiver fault orientations based on the focal mechanism and its uncertainty. Many of the aftershocks in the shadows consistently show a Coulomb stress decrease on the planes of their focal mechanisms. These results imply that aftershocks do occur in stress shadows, many on fault planes receiving a decrease in static Coulomb stress, contrary to Hypotheses 1 and 2. We test Hypothesis 3 by investigating the modeled dynamic stress changes on the individual event focal mechanisms. Preliminary results show that while the amplitude of the dynamic Coulomb stress change is generally lower on the aftershock nodal planes than on the planes of background events, the amplitude of the dynamic normal stress change is often 20%-100% higher. This suggests a dynamic triggering mechanism related to changing fault strength.